A Theoretical Study of the Structure and Spectra of Nitric Acid Hydrates Crystals

Quantum chemical study of atmospheric aggregates: HCl•HNO3•H2SO4

HCl, HNO3 and H2SO4 are implicated in atmospheric processes in areas such as polar stratospheric clouds in the stratosphere. Ternary complexes of HCl, HNO3 and H2SO4 were investigated by ab initio calculations at B3LYP level of theory with aug-cc-pVTZ and aug-cc-pVQZ basis sets, taking into account basis set superposition error (BSSE). The results were assessed in terms of structures (five hexagonal cyclic structures and two quasi-pentagonal cyclic structures), inter-monomeric parameters (all ternary complexes built three hydrogen bonds), energetics (seven minima obtained), infrared harmonic vibrational frequencies (red shifting of complexes from monomers), and relative stability of complexes, which were favorable when the temperature decreases under stratospheric conditions, from 298 K to 188 K, and in concrete, at 210 K, 195 K and 188 K.

Review of heterogeneous photochemical reactions of NOy on aerosol - A possible daytime source of nitrous acid (HONO) in the atmosphere

As an important precursor of hydroxyl radical, nitrous acid (HONO) plays a key role in the chemistry of the lower atmosphere. Recent atmospheric measurements and model calculations show strong enhancement for HONO formation during daytime, while they are inconsistent with the known sources in the atmosphere, suggesting that current models are lacking important sources for HONO. In this article, heterogeneous photochemical reactions of nitric acid/nitrate anion and nitrogen oxide on various aerosols were reviewed and their potential contribution to HONO formation was also discussed. It is demonstrated that HONO can be formed by photochemical reaction on surfaces with deposited HNO3, by photocatalytic reaction of NO2 on TiO2 or TiO2-containing materials, and by photochemical reaction of NO2 on soot, humic acids or other photosensitized organic surfaces. Although significant uncertainties still exist in the exact mechanisms and the yield of HONO, these additional sources might explain daytime observations in the atmosphere.

Theoretical study on hydrogen-bond effects in IR spectra of high- and low-temperature phases of nitric acid dihydrate

The low- and high-temperature phases (alpha and beta, respectively) of solid nitric acid dihydrate (NAD) are studied in depth by DFT methods. Each phase contains two types of complex structures (H(3)O(+)) x (H(2)O), designated A and B, with different hydrogen-bonding (HB) characteristics. The theoretical study reveals that type A complexes are weakly bound and could be described as (H(3)O)(+) and H(2)O aggregates, with decoupled vibrational modes, whereas in type B structures the proton is situated close to the centre of the O...O bond and induces strong vibrational coupling. The proton-transfer mode is predicted at quite different wavenumbers in each complex, which provides an important differentiating spectral feature, together with splitting of some bands in beta-NAD. Theoretical spectra are estimated by using two GGA parameterizations, namely, PBE and BLYP. The potential-energy surface for each type of HB in NAD is also studied, as is the spectral influence of displacement of the shared H atom along the O-O bond. The results are compared to literature infrared spectra recorded by different techniques, namely, transmission and reflection-absorption, with both normal and tilted incident radiation. This work provides a thorough assignment of the observed spectra, and predictions for some spectra not yet available. The usefulness of high-level theoretical calculations as performed herein to discriminate between two phases of a solid crystal is thus evidenced.

FTIR spectroscopy combined with quantum chemical calculations to investigate adsorbed nitrate on aluminium oxide surfaces in the presence and absence of co-adsorbed water

Surface reactions of nitrogen oxides with aluminium oxide particles result in the formation of adsorbed nitrate. Specifically, when alpha-Al(2)O(3) and gamma-Al(2)O(3) particles are exposed to gas-phase NO(2) and HNO(3) adsorbed nitrate forms on the surface. In this study, Fourier transform infrared (FTIR) spectroscopy is combined with quantum chemical calculations to further our understanding of the adsorbed nitrate product on aluminium oxide particle surfaces in the presence and absence of co-adsorbed water at 296 K. FTIR spectra of adsorbed nitrate on alpha-Al(2)O(3) and gamma-Al(2)O(3) particles are interpreted using calculated vibrational frequencies of nitrate coordinated to binuclear Al oxide cluster models. Comparison of the calculated and experimental vibrational frequencies of adsorbed nitrate establishes different modes of coordination (monodentate, bidentate and bridging) of the nitrate ion to the surface in the absence of adsorbed water. In the presence of co-adsorbed water, the nitrate ion becomes fully solvated, as shown by a comparison of the experimental nitrate infrared spectra as a function of relative humidity with the calculated nitrate vibrational frequencies for binuclear Al cluster compounds which contain both coordinated nitrate ions and water molecules. These calculations also suggest that adsorbed water can displace nitrate from direct coordination to the surface, leading to an outer-sphere nitrate adsorption complex as well as an inner-sphere complex. Furthermore, the relative humidity dependence of the spectra suggest that water does not evenly wet the surface even at high relative humidity, as there are open or bare surface sites where nitrate ions are not solvated. Besides adsorbed mondendate, bidendate, bridging and solvated nitrate, the presence of ion bound nitrate ion, partially solvated nitrate, molecular nitric acid, hydronium ion and H(3)O(+):NO(3)(-) ion pairs on the oxide surface are also discussed.

Orientation Effects on Nitric Acid Dihydrate Films

An investigation of orientation effects in films of nitric acid dihydrate (NAD) is presented, based on a systematic study of transmission and reflection-absorption infrared (RAIR) spectra of samples of varying thickness. The samples are prepared by vapor deposition on Ge (for transmission spectroscopy) and on Al substrates (for RAIR spectroscopy) at 175 K to produce crystalline alpha-NAD films. Transmission spectra were recorded at normal incidence, and RAIR spectra were recorded at a grazing angle of 75 degrees, with polarized radiation. The observed spectra are compared with predictions of a classical Fresnel model, to test the available optical indices of NAD, which are of great importance for the accurate interpretation of data from remote sensing measurements. Whereas the procedure yields satisfactory results for transmission and s-polarized RAIR spectra, it is found that the agreement is not acceptable for p-polarized RAIR spectra. An explanation is suggested in terms of a preferential alignment of the films, with the (10-1) crystallographic plane of the crystal situated parallel to the substrate. The infrared activity of a band at approximately 1170 cm(-1) is explained in terms of a preferential orientation of the crystal domains in the film.

New Experimental and Theoretical Approach to the Heterogeneous Hydrolysis of NO2: Key Role of Molecular Nitric Acid and Its Complexes

Although heterogeneous chemistry on surfaces in the troposphere is known to be important, there are currently only a few techniques available for studying the nature of surface-adsorbed species as well as their chemistry and photochemistry under atmospheric conditions of 1 atm pressure and in the presence of water vapor. We report here a new laboratory approach using a combination of long path Fourier transform infrared spectroscopy (FTIR) and attenuated total reflectance (ATR) FTIR that allows the simultaneous observation and measurement of gases and surface species. Theory is used to identify the surface-adsorbed intermediates and products, and to estimate their relative concentrations. At intermediate relative humidities typical of the tropospheric boundary layer, the nitric acid formed during NO2 heterogeneous hydrolysis is shown to exist both as nitrate ions from the dissociation of nitric acid formed on the surface and as molecular nitric acid. In both cases, the ions and HNO3 are complexed to water molecules. Upon pumping, water is selectively removed, shifting the NO(3-)-HNO3(H2O)y equilibria toward more dehydrated forms of HNO3 and ultimately to nitric acid dimers. Irradiation of the nitric acid-water film using 300-400 nm radiation generates gaseous NO, while irradiation at 254 nm generates both NO and HONO, resulting in conversion of surface-adsorbed nitrogen oxides into photochemically active NO(x). These studies suggest that the assumption that deposition or formation of nitric acid provides a permanent removal mechanism from the atmosphere may not be correct. Furthermore, a potential role of surface-adsorbed nitric acid and other species formed during the heterogeneous hydrolysis of NO2 in the oxidation of organics on surfaces, and in the generation of gas-phase HONO on local to global scales, should be considered.

The crystallization kinetics and morphology of nitric acid trihydrate

Crystallization kinetics of stable and metastable nitric acid trihydrate (NAT) were investigated by time dependent X-ray powder diffraction (XRD) measurements. Kinetic conversion curves were evaluated adopting the Avrami model. The growth and morphology of the respective crystallites were monitored in situ on the cryo-stage of an environmental scanning electron microscope (ESEM) under a partial pressure of nitrogen gas (0.3 Torr, 40 Pa). The results show a close relationship between the presence of ice in the sample and the crystallization mechanism of NAT, which results in different shapes and sizes of NAT crystal particles.

Theoretical Calculations of Refractive Indices and Optical Effects in Spectra of Nitric Acid and Nitric Acid Monohydrate Crystals

The theoretical infrared refractive indices of two systems related to atmospheric research, nitric acid (NA) and nitric acid monohydrate (NAM) crystals, have been computed using a methodology based on first-principles. The effects of lack of coherence in the infrared beam in RAIR and transmission spectra have also been treated using a model based on classical optics. The optical constants of NA crystals are presented for the first time; the results on NAM are compared to empirical values previously published with good general agreement. With the optical constants of NA, polarized reflection-absorption infrared spectra are predicted and compared to experimental spectra recorded also for the first time, for a set of varying film thickness. The global agreement is satisfactory. The effects of a number of experimental factors in transmission spectra of NAM are assessed, in an attempt to explain observed differences among experimental spectra. It is concluded that the spectral disparities are probably due to differences in the nature of the samples.

Infrared Spectrum of Nitric Acid Dihydrate: Influence of Particle Shape

In situ Fourier transform infrared (FTIR) extinction spectra of airborne alpha-NAD microparticles generated by two different methods were recorded in the large coolable aerosol chamber AIDA of Forschungszentrum Karlsruhe. The extinction spectrum of alpha-NAD crystals obtained by shock freezing of a HNO3/H2O gas mixture could be accurately reproduced using Mie theory with published refractive indices of alpha-NAD as input. In contrast, Mie theory proved to be inadequate to properly reproduce the infrared extinction spectrum of alpha-NAD crystals which were formed via homogeneous nucleation of supercooled HNO3/H2O solution droplets, evaporating slowly on a time scale of several hours at about 195 K. Much better agreement between measured and calculated extinction spectra was obtained by T-matrix calculations assuming oblate particles with aspect ratios greater than five. This indicates that strongly aspherical alpha-NAD crystals are obtained when supercooled nitric acid solution droplets freeze and grow slowly, a process which has been discussed as a potential pathway to the formation of crystalline polar stratospheric cloud (PSC) particles.